Cyanide solutions are widely used in a number of chemical processes. A common use of cyanide is in the leaching of gold. The chemistry by which cyanide leaches gold may be expressed as follows: EQU 4Au+8NaCN+O.sub.2 +2H.sub.2 O-&gt;4NaAu(CN).sub.2 +4NaOH (1)
Gold is usually present in very low concentrations in naturally occurring ores and in concentrates derived from such ores. Typical gold concentrations are in the range of from about 1 g/tonne for some ores to about 1000 g/tonne for some concentrates. In the leaching process, cyanide is typically added to ores or concentrates at elevated pH to keep cyanide in solution and to thereby prevent the formation and evolution of highly toxic HCN gas. To help maximize the efficiency of leaching, cyanide is typically added in excess of the stoichiometric amount required for leaching in accordance with reaction 1. The excess cyanide is required in part because cyanide typically reacts with other minerals, is oxidized or volatilizes from the system.
Following leaching, gold may be recovered by a number of processes, such as zinc cementation or carbon adsorption, leaving a barren solution. The presence of excess cyanide in the barren solution at the end of the gold leaching and recovery operation creates a disposal problem for gold leaching plants. A variety of approaches may be taken to address this problem. The cyanide may be discharged to the environment if the cyanide concentration is sufficiently low. The cyanide may be destroyed using a chemical or biological treatment method, such as known methods of SO.sub.2 /air treatment, alkaline chlorination, biological oxidation, hydrogen peroxide treatment or Caro's acid treatment. The cyanide may be recovered for recycle by known methods such as AVR (acidification, volatilization and re-neutralization), AFR (acidification, filtration and reneutralization), or MNR (Metallgeselshaft Natural Resources) processes, the Cyanisorb.TM. process, or the Augment.TM. process.
The AVR process has been of interest to gold processors for a long time. The process involves addition of acid to a waste cyanide solution followed by volatilization of HCN gas, reneutralization and scrubbing of volatile HCN from the stripping air, in accordance with the reactions 2, 3 and 4.
Acidification: EQU 2CN.sup.- +H.sub.2 SO.sub.4.fwdarw.2HCN(aq)+SO.sub.4.sup.2- (2)
Volatilization: ##EQU1##
Reneutralization: EQU 2HCN(g)+2NaOH(aq).fwdarw.2NaCN(aq)+2H.sub.2 O (4)
NaCN recovered from the reneutralization step of an AVR process may be returned to a leaching process. The AVR process may be particularly useful when cyanide is present as metal complexes such as copper cyanides. As shown in equation 5, two of the three cyanides in the copper cyanide complex may be recovered by this method while copper is recovered as a CuCN precipitate. EQU Cu(CN).sub.3.sup.2- +H.sub.2 SO.sub.4.fwdarw.2HCN(aq)+SO.sub.4.sup.2- +CuCN(s) (5)
The AVR method suffers from a number of potentially important drawbacks, including volatilization of dangerous HCN gas and inefficiencies that may raise costs. Volatile HCN is acutely toxic, which raises important safety concerns, particularly in areas where there is a higher risk of leaching plant disruption due for example to power outages. HCN is a soluble acid that is difficult to strip using air so that the size and cost (capital and operating) for volatilization operations may be significant. Also, the large volumes of gas typically used for volatilization must be scrubbed with NaOH to ensure good cyanide recovery and to minimize HCN loss in any discharged `tail` gas, a process that may further raise costs.
A second known method of dissolved cyanide recovery is the MNR process. This process differs slightly from the AVR process in that NaSH (sodium hydrosulfide) is added during acidification. The NaSH is thought to maximize cyanide recovery by converting base metal cyanides to metal sulfides, as shown in equations 6 through 9.
Acidification and Sulfidization EQU 2CN.sup.- +H.sub.2 SO.sub.4.fwdarw.2HCN(aq)+SO.sub.4.sup.2- (6) EQU 2Cu(CN).sub.3.sup.2- +5/2H.sub.2 SO.sub.4 +NaSH.fwdarw.Cu.sub.2 S+6HCN(aq)+5/2SO.sub.4.sup.2- +Na.sup.+ (7)
Volatilization ##EQU2##
Reneutralization EQU 2HCN(g)+2NaOH(aq).fwdarw.2NaCN(aq)+2H.sub.2 O (9)
The MNR process suffers from many of the same drawbacks as the AVR process, since HCN is volatilized and scrubbed.
It is an object of the invention to provide a process for the recovery of cyanide from aqueous solutions that my be used as an alternative to known processes such as volatilization and reneutralization of HCN as part of AVR or MNR processes.